The present invention relates to a method of controlling the oxide formation, such as nickel plated copper alloy, on a metal substrate so as to allow for the selective plating thereof by a precious metal, such as gold or an alloy thereof.
The selective plating thereof is achieved by selectively removing a plating resist covering said substrate. Such removal is accomplished by matching or coupling the laser wavelength, preferably one operating in the U.V. range, such as an excimer laser, with the plating resist and metal substrate.
The laser energy density required to achieve such oxide control must exceed the threshold energy density needed to achieve complete removal of such resist in a single focussed laser beam.
A preferred embodiment of this invention is the selective plating of electrical terminals. Typically, such terminals are stamped and formed from metal strip and are attached to a carrier strip which is useful for strip feeding the terminals through successive manufacturing operations. One necessary manufacturing operation involves plating; i.e., electroplating the electrical contact surfaces of the strip fed terminals with precious metal or semi-precious metal, such as gold or alloys thereof. Such metals are characterized by good electrical conductivity and little or no formation of oxides that reduce said conductivity. Therefore these metals, when applied as plating, will improve conductivity of the terminals. However, the high cost of these metals has necessitated precision deposition on the contact surfaces of the terminals, and not on surfaces of the terminals on which plating is not necessary. Unfortunately, a necessary criteria to good electrical contact is the preparation of an essentially oxide free metal substrate upon which the precious or semi-precious metal is plated. Due to the unique requirements of electrical terminals, and the need to precisely control the plating thereof, certain mechanical and/or chemical means cannot be used. For example, U.S. Pat. No. 4,063,063 to Funck et al. teaches a method of descaling a metal surface by the combination of a sweeping laser beam to detach an adhering oxide film, followed by removal of such oxide film, such as by mechanical, chemical or electrolytic means. Such patent appears to be limited to the treatment of a broad surfaced product, such as a continuous strip. U.S. Pat. No. 4,368,080 to Langen et al. teaches a method of removing rust from a metal surface by focusing a laser beam on such surface in a controlled environment.
It is now known that laser beams have been employed to improve metal substrate surfaces for subsequent plating. In co-pending application, Ser. No. 133,779, now U.S. Pat. No. 4,832,798, and owned by the assignee herein, a technique is taught whereby the porosity of a nickel plated substrate is significantly reduced by a laser beam to permit a reduction in the level of precious metal plating needed on such nickel to produce a good contact surface.
U.S. Patent No. 4,348,263 to Draper et al. and directed to a process for surface melting of a substrate prior to plating, teaches a method of making an electrical contact by the steps of applying a first protective layer to a substrate, subjecting said protective layer and a portion of said substrate to melting by means of an electron beam or laser prior to the deposition. A related work by Draper, published in the Gold Bulletin, 1986, 19, entitled "Laser Surface Alloying of Gold," contains an illustrated showing on the mechanism of laser surface alloying by the use of focussed laser pulsing.
Laser beams have also been used as a means to remove or ablate a substrate coating whereby to expose such substrate. By way of background, ablation is defined as the process of removal of a part such as by melting or vaporization. The laser is the mechanism by which one may achieve the selective melting or vaporization. By the use of different lasers, particularly ones utilizing a broadly differing wavelength, the general process of laser ablation is affected. For example, by the use of an excimer laser, operating in the U.V. range, coupled with a resist essentially transparent to the wavelength of such laser, and a metal substrate from which the resist is to be removed and which absorbs such wavelength in co-pending application, Ser. No. 180,417, now U.S. Pat. No. 4,877,644, a different laser assisted material removal process is observed. The latter process, which is more energy efficient, may be termed "patch blowoff." That is, rather than melting or vaporizing the resist, an interesting phenomena occurs at the resist/substrate interface resulting in the overlying resist being blownoff, essentially as solid particles. Thus, while laser ablation has been broadly used to define any process where a laser is used to assist in a material removal process, it will be appreciated that the certain parameters applied will render the various approaches quite distinctive.
Returning now to the broad concept, it can be acknowledged that selective removal of a resist may be accomplished by the technique known as laser ablation. Reports have appeared in the literature regarding attempts at laser ablation of polymer coatings on metals, and regarding methods of multi-shot removal of polymer coatings on non-metals. R. Srinivasan et al, in the JAP 59, 3861 (1986) and JVST, B1, 923 (1983) describe, for example, the use of excimer laser wavelengths which are strongly absorbed directly in the polymer itself to achieve removal of polymer by chemical bond-breaking or heating to vaporization, or a combination of both. However, the authors found that polymer ablation occurs when the laser light is absorbed within about the first 0.2 micron or less of the polymer surface. Then only that polymer material within the characteristic absorption depth was removed. In order to remove a thicker polymer film, such as is necessary for most electroplating requirements, multiple laser shots would be required. The use of multiple shots is much less desirable than single shot removal. One problem associated with the method of Srinivasan et al, wherein the laser light is directly absorbed in the polymer, is that choosing a laser wavelength too strongly absorbed in the polymer necessarily implies a small absorption depth and small thickness removed. On the other hand, choosing a wavelength too weakly absorbed in the polymer precludes depositing sufficient energy per unit volume of polymer to achieve ablation. The compromise value between these extremes dictates that no more than about 0.3 micron per pulse can be removed in the best case. Cole et al, in Mat. Res. Soc. Symp. Proc. 72, 241 (1986), concur with Srinivasan et al in this finding. The above process represents the current state of the art on excimer laser ablation of polymers.
In U.S. Pat. No. 4,671,848 to Miller et al, a method for the removal of a dielectric coating from a conductor, by means of a focused, high energy radiation source, is taught. More particularly, in said method a laser source is focused to a point above the coating which results in a plasma or ionized region being formed. As a consequence, the coating is removed in a preselected region on the underlying conductor. In other words, the laser ablation depends on absorption of laser light by ionized air or other plasma and transmission to the dielectric. A difficulty of this method is the ability to control and adjust the air breakdown so as to ensure there is no damage to the conductor, i.e. underlying substrate, and to achieve removal of the residual layer. Another difficulty is that only a small area corresponding to the tight focus region can be removed on each shot. Miller et al state that extended areas are to be ablated by multiple shots while moving the workpiece, or the laser focus.
Thus, whether dealing with the conventional ablation process, or the more recently defined practice of "patch blowoff," the concerns have been primarily with the most effective or efficient means to achieve the resist removal. That is, there appears to have been little concern with improving the surface adhesion properties of the metal substrate upon which selective plating is required.
The present invention accomplishes this goal by carefully coordinating the energy density of the excimer laser beam with that of the threshold energy necessary to completely remove the resist in the focussed area by a single shot or laser pulse.